The invention relates to an epitaxial base heterojunction bipolar transistor and a method of fabricating the same.
Epitaxial SiGe-base bipolar transistors have been known in the art as disclosed in IEEE Electron Devices Letters, Vol. 12, No. 4, April 1991 pp166-168 John D. Cressler, at al. "Sub-30-ps ECL Circuit Operation at Liquid-Nitrogen Temperature Using Self-Aligned Epitaxial SiGe-Base Bipolar Transistors".
FIG. 1 illustrates a structure of the conventional self-aligned epitaxial SiGe-base bipolar transistor. The conventional bipolar transistor has a p.sup.- -type silicon substrate 1. An n.sup.+ -type buried layer 2 is formed on the p.sup.- -type silicon substrate 1. The p.sup.- -type silicon substrate 1 and the n.sup.+ -type buried layer 2 include two deep trench isolation regions containing polycrystalline silicon regions 21. The trench isolation regions are closed at those bottoms by channel stopper p.sup.+ -type layers 3 respectively. The trench isolations are covered with a silicon oxide layer 5 formed by the normal process such as the selective oxidization. The n.sup.+ -type buried layer 2 is connected to a collector contact through an contact hole in the silicon oxide layer 5. The silicon oxide layer 5 surrounds an n-type silicon region 4 which serves as a collector region.
The transistor has a p.sup.+ -type SiGe epitaxial base layer 7 which is deposited by using an ultrahigh-vacuum chemical vapor deposition low-temperature epitaxy. Thin lightly doped spacer layers (not shown) are formed at both the emitter and collector sides of the base layer 7 to decrease junction fields and parasitic capacity. The epitaxial base layer 7 formed by using the low-temperature epitaxy has a small thickness thereby providing high gain at low temperature while a cut-off frequency f.sub.T of the device is improved. The epitaxial base layer 7 permits minimizing a gate delay. The base layer 7 is made to directly contact with a base contact made of metals such as an aluminum system.
The transistor has an n.sup.+ -type polycrystalline silicon region 13 on the epitaxial base layer 7. The n.sup.+ -type polycrystalline silicon region 13 serves as an emitter region. The n.sup.+ -type polycrystalline silicon region 13 is surrounded by the silicon oxide region. An emitter contact is provided on the n.sup.+ -type polycrystalline silicon region 13.
FIG. 2 illustrates final vertical profiles of dopant concentrations in the n.sup.+ -type polycrystalline silicon region 13 serving as the emitter region and the epitaxial base layer 7 as well as the mole-percent of germanium in the epitaxial base region 7. The profile data is obtained by secondary ion mass spectrometry (SIMS) analysis. The base width is approximately 59 nm with a peak germanium concentration of approximately 9 percent. The emitter-base p-n junction exists in the vicinity of the heterojunction.
Such heterojunction bipolar transistors (HBT) are, however, engaged with disadvantages such as a formation of a parasitic electron barrier which effectively affects the transport of electrons across the epitaxial base layer 7. For instance, heterojunction bipolar transistors of III-V group heterojunction such as AlGaAs/GaAs are subjected to no heat treatment for diffusion after a crystal growth by using a molecular beam epitaxy or the like because a heat treatment is undesirable for such AlGaAs/GaAs.
In contrast, a silicon based heterojunction bipolar transistor such as a SiGe heterojunction bipolar transistor including a polycrystalline emitter possesses a good common-emitter current gain h.sub.FE. Such a transistor requires a heat treatment for impurity diffusion from polycrystalline silicon to monocrystalline silicon. This results in a sufficiently high current gain h.sub.FE which provides a sufficiently small emitter charge/discharge time thereby permitting such transistor to have the high cut-off frequency f.sub.T as compared with a transistor having the same thickness base.
It is, however, considerable that the difference between thermal diffusion rates of boron and germanium is very large, typically two digits. This permits the out-diffusion of boron which causes potential barriers at the emitter collector sides of the base layer 7. The p-n junctions at the emitter and collector sides of the base layer 7 exist in the silicon regions respectively. Especially, the transport of minority carriers, or holes are substantially affected by the out-diffusion. The effects of the out-diffusion of boron on the device performance are disclosed in IEEE Technical Digest of IEDM (1989) p639.
The conventional device is engaged with following disadvantages. The device may have uneven heat distributions within a wafer during the heat treatment for dopant diffusion and uneven thickness of the base layer by the epitaxial growth. The p-n junction position of the emitter side may, therefore, be varied through the heterojunction of silicon and silicon germanium. This result is that the base emitter voltage VBE for a constant collector current is also variable.